I'm confused here: "The brake handles for my disk brakes became very loose four months after I bought the bike. The bike shop bled the brakes and put new oil in them, which sorted it.". If your levers are becoming loose, bleeding your brakes will do nothing, it's a case of tightening a bolt or two. Sounds like your LBC just scammed you. Even in very wet, dirty conditions hydro brakes only need bleeding once every year or two.
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cmannett85Oct 13 '11 at 7:03

I did feel scammed, particularly when they said it wasn't covered by the waranty. But, the reason I was given was there was not enough pressure in the hyrolics to engage the brakes and this was through wear and tear of use.
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RichardOct 13 '11 at 8:22

What make and model hydraulics disk brakes do you have? I have Shimano LX (before SLX), commuting for 3 years at about 5000km a year, and ok, I should have serviced them by now as a precaution, but they still feel fine to me. I also asked my LBS about a year ago and they said they feel fine too (they're a good shop). I've been through about a half dozen sets of pads in that time. Although I might try bleeding them now you've reminded me ;-)
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Jason SOct 17 '11 at 22:41

5 Answers
5

In general, hydraulic brakes should only need bleeding when the system has been "opened up" for some reason, or when a leak has either allowed air in or allowed enough brake fluid to drain out that the master cylinder is pumping air. The "need to bleed" indicates a problem with the brakes.

On autos, hydraulic brakes should be drained and new fluid installed about every 50,000 miles (though this is rarely done). Comparable mileage on a bike would probably be 5,000-10,000 miles.

If the brake response gets mushy, also check the brake pads. Personal experience: I saw no need in bleeding the brakes before changing the brake fluid (on a DOT system). Maybe your system is not sufficiently sealed?

The need to exchange the fluid depends on the type that is used in your brakes. And this should be self-evident: Do not, under any circumstances, change the type of fluid used in your brake! It will most certainly degrade the gaskets.

DOT

If your brakes use DOT (classified by the US Department of Transportation) brake fluid, i.e. "DOT 5.1", the fluid should be exchanged every 1-2 years [1]. With time, the fluid absorbs water, affecting its compressibility. This is by design, since non-absorbed water would affect the compressibility even more.

Mineral Oil

Mineral oil, however, does not need to be changed so regularly. Unless you open the system, the oil stays good for up to 6-7 years [2].

Note that the proper generic term is "brake fluid" or "hydraulic fluid" -- the DOT fluid is not oil, but a glycol/alcohol mixture, similar to antifreeze, or, in the newer DOT 5 version, a silicone fluid. You definitely DO NOT want to put any kind of oil in a system designed for DOT fluid, and probably you shouldn't put DOT fluid in a system designed for mineral oil (and certainly not mix them). (And note that there's a distinction between simply "bleeding" brakes and exchanging the brake fluid.)
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Daniel R HicksOct 15 '11 at 12:36

+1 for mineral oil. I have some 2004 Shimano disk brakes that have never needed bleeding. The only time they have been bled was when building a new bike 2 years ago and wanted to learn how to bleed them :)
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si618Apr 30 '12 at 3:57

Yeah, every few years the fluid should be replaced, since it slowly absorbs moisture, etc. The same thing is true on cars. Shouldn't need "bleeding" before that, though.
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Daniel R HicksOct 13 '11 at 21:26

Typical hydraulic disc brakes use an 'open' design to allow for automatic pad adjustment.
They have a small reservoir at near the brake lever for 'refilling' the extra brake fluid that is needed when the brake pad needs to be pushed a little closer to the disk in order to compensate the brake pad wear.

This type of design leads to less hassle in respect to pad adjustment, but has one main disatvantage: as more and more fluid gets pumped from the reservoir towards the brake cylinders, the oil gets replaced by air. Now if the bikes position changes from the regular upright position to e.g. upside down, air bubbles can slip to the brake hose and that causes the spongy feeling in the brake levers.

So you don't necessarily have to open up the system to get air in, that's quasi by design.
With a little luck you can pump the air back to the reservoir, but normally you need to bleed the system.

There should always be enough fluid in the reservoir such that air is never, in normal operation, pumped into the lines. You do have a point, though, that placing the bike upside down and then operating the brake lever could allow air into the lines. Air should not get in if the bike is simply inverted for a reasonably short period of time.
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Daniel R HicksOct 17 '11 at 17:31

I think this is what I have done. I inverted the bike to replace a flat and oil the chain, the brake several times. Now the brake feel really loose. What can I to fix this? Do I need to take it back for "bleeding"? (I've never know such a thing until now).
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Wai Yip TungOct 19 '11 at 0:07

As you use the brakes and the pads wear down, the oil will heat up and expand. This pushes oil back up into the reservoir, and when the oil cools off the pistions will suck away from the rotors.

Look at the brakes and you'll notice the pads probably aren't floating against the rotor as they should be, but when you squeeze the handle most of the motion is moving the pads to the rotor. When pads and rotors are new this doesn't happen because the pads are thick.

Bleeding doesn't help this, as it's normal as pads wear. The pumping you do during braking doesn't move enough fluid back into the lines to get the pads to set back next to the rotors.

The quick, easy and proper way to fix is to take the wheel off, and continuously quickly pump the handle until the pads meet in the middle of the caliper. When you can't squeeze any more then you've loaded the lines. Then using a clean flat tip driver, gently pry them open just slightly enough to get the rotor back in. Your brakes will feel like new and should feel good for awhile longer. The more your pads wear however, the more you will have to perform this little reset.